Facts which might assist to explain the origin of sex have been sought after from very early times, and have been also placed in very different lights. The result on every occasion, when this subject has been discussed, has been always a wide difference of opinion. People have, in consequence, been induced to fall back upon theories of different sorts, theories which have for varying periods, sometimes long, sometimes short, been accepted as of some assistance towards a scientific explanation. In all the theories which have been propounded, the sex has been regarded as already determined in the ovum, or else the origin of the sex has been assigned to some early stage of the development. The earliest statements extend back into the ages of myth and fable, in consequence of which any exact comparison of them is not an easy task. All the different manuals which deal with the present question touch upon these early views, and for this reason I am unwilling entirely to ignore them here. I shall, accordingly, select a few of the more important for mention. The reproductive glands of the two sexes were supposed to contain generative matter distributed in such a way, on the right and left, that the right ovary and the right testicle contained the generative secretions for the production of the males, and the left ovary and left testicle those which produced females. It is immediately evident that, according to this theory, it was impossible to exercise any influence over the sex of the future individual. This primitive theory is ever cropping up anew, always to be again rejected. Of various other theories of the same kind, only those deserve any attention which rest upon some basis of fact. Accordingly, recourse has been had to statistics, and an attempt has been made to reach, from the figures which they furnish, some certainty respecting which sex was the more numerous, and what should be concluded to be the cause of the greater prevalence of the one sex or the other. The fact was, however, apparently overlooked that the available statistics, though in many respects of the highest scientific value, could be of real significance only when the numbers were gathered from widely distributed peoples amongst whom there was none of that wandering about the world which characterizes modern society. I am at the same time unwilling to omit data, resting upon numbers which have been gathered from statistics, and are not without value for the determination of many important questions. Ploss has in this way shown that in favorable years, when food was cheap, the births showed an excess of girls. Under unfavorable circumstances, more male individuals were born. A comparison of statistics, however, soon led to another theory, which culminated in this result, that in all countries an excess of male individuals was born.